Method For Production Of 1,6-Hexanediol With A Purity In Excess of 99.5%

ABSTRACT

The present invention provides a process for preparing 1,6-hexanediol by hydrogenating dialkyl adipates, alkyl 6-hydroxycaproates, 1,4-cyclohexanedione and 4-hydroxycyclohexan-1-one as ester mixtures comprising impurities, by
         a) freeing resulting the esterification mixture of excess alcohol and low boilers in a first distillation stage (alcohol removal),   b) carrying out a separation of the bottom product in a second distillation stage into an ester fraction substantially free of 1,4-cyclohexanediols and a fraction comprising at least the majority of the 1,4-cyclohexanediols,   c) catalytically hydrogenating the ester fraction substantially free of 1,4-cyclohexanediols (ester hydrogenation) and   d) in a purifying distillation stage, obtaining 1,6-hexanediol from the hydrogenation effluent in a manner known per se,
 
which comprises selectively hydrogenating the ester mixture before stage a) and/or before stage b) (purifying hydrogenation).

The invention relates to a process for preparing 1,6-hexanediol in apurity of >99.5% from mixtures comprising dialkyl adipates, alkyl6-hydroxycaproates, 1,4-cyclohexanedione and 4-hydroxycyclohexan-1-one.

1,6-Hexanediol constitutes a sought-after monomeric building block whichis used predominantly in the polyester and polyurethane sector. In theseapplications, 1,4-cyclohexanediol in the 1,6-hexanediol is undesired.

Suitable processes for preparing 1,6-hexanediol are described in DE-A196 07 954 and DE-A 196 07 955. In these processes, dicarboxylic acidsolution (DCS) is esterified initially with a C₁-C₁₀-alkanol and theresulting esterification mixture, after removal of excess alcohol andother low boilers, is separated by distillation. An ester fraction isobtained which is substantially free of 1,4-cyclohexanediols.

From this ester fraction, 1,6-hexanediol with a purity of at least 99%is prepared by final hydrogenation (ester hydrogenation).

Even though the C₆ ester mixture prepared by this process issubstantially free of 1,4-cyclohexanediols, 1,4-cyclohexanediols arefound in the hydrogenation effluent depending on the quality of thedicarboxylic acid solution used and cannot be removed fully bydistillation and therefore appear at 0.05-0.5% in the pure1,6-hexanediol.

Reactants for the processes, described in DE-A 196 07 954 and DE-A 19607 955, for the preparation of 1,6-hexanediol are the aqueous solutionsof carboxylic acids which are formed as by-products in the oxidation ofcyclohexane to cyclohexanol and cyclohexanone (cf. Ullmann'sEncyclopedia of Industrial Chemistry, 5th Ed., 1987, Vol. A8, p. 49),referred to hereinbelow as dicarboxylic acid solution (DCS). Thesedicarboxylic acid solutions contain (calculated without water in % byweight) generally between 10 and 40% adipic acid, between 10 and 40%6-hydroxycaproic acid, between 1 and 10% glutaric acid, between 1 and10% 5-hydroxyvaleric acid, between 1 and 5% 1,2-cyclohexanediols,between 1 and 5% 1,4-cyclohexanediols, between 2 and 10% formic acid anda multitude of further mono- and dicarboxylic acids, esters, oxo and oxacompounds whose individual contents generally do not exceed 5%. Examplesinclude acetic acid, propionic acid, butyric acid, valeric acid, caproicacid, oxalic acid, malonic acid, succinic acid, 4-hydroxybutyric acidand γ-butyrolactone.

A more precise analysis of the dicarboxylic acid solution revealed1,4-cyclohexanedione and 4-hydroxycyclohexan-1-one as furtheringredients at from 0.01 to 2% by weight.

Both substances are removed only insufficiently after esterification anddistillative purification of the esters. 1,4-Cyclohexanedione and4-hydroxycyclohexanone which get into the final hydrogenation of theester mixture to hexanediol (ester hydrogenation) are hydrogenated thereunder the conditions of the ester hydrogenation to 1,4-cyclohexanedioland lead to a contamination which can be removed only with high yieldlosses of the hexanediol product.

It is therefore an object of the present invention to provide a simpleand inexpensive process which affords 1,6-hexanediol in purer form from1,4-cyclohexanedione- and 4-hydroxycyclohexan-1-one-containingdicarboxylic acid solution without the yield of 1,6-hexanediol falling.

It has now been found that, surprisingly, this object is achieved whenthe ester mixture is subjected to a hydrogenation before a distillation.

The present invention provides a process for preparing 1,6-hexanediolwith a purity of >99% by hydrogenating dialkyl adipates, alkyl6-hydroxycaproates and 1,4-cyclohexanedione and4-hydroxycyclohexan-1-one as ester mixtures comprising impurities, by

-   -   a) freeing the resulting esterification mixture of excess        alcohol and low boilers in a first distillation stage (“alcohol        removal” hereinbelow),    -   b) carrying out a separation of the bottom product in a second        distillation stage into an ester fraction substantially free of        1,4-cyclohexanediols and a fraction comprising at least the        majority of the 1,4-cyclohexanediols,    -   c) catalytically hydrogenating the ester fraction substantially        free of 1,4-cyclohexanediols (“ester hydrogenation” hereinbelow)        and    -   d) in a purifying distillation stage, obtaining 1,6-hexanediol        from the hydrogenation effluent in a manner known per se,        which comprises selectively hydrogenating the ester mixture        before stage a) and/or before stage b), but preferably before        stage b), of the distillation for the 1,4-cyclohexanediol        removal (“purifying hydrogenation” hereinbelow).

During the inventive selective purifying hydrogenation of the1,4-cyclohexanedione and 4-hydroxycyclohexan-1-one present in the estermixture, the adipic acid and alkyl 6-hydroxycaproates which are likewisepresent are not hydrogenated to alcohols. This prevents them from beinglost as bottom product in the distillation (stage b)) which preferablyfollows the purifying hydrogenation and the hexanediol yield fromfalling drastically.

Surprisingly, the amount found of 1,4-cyclohexanediols which cannot beremoved 1,6-hexanediol is distinctly reduced or no longer present at allafter the hydrogenation, without the 1,6-cyclohexanediol yield havingfallen.

Dialkyl adipates to be used in accordance with the invention are inparticular esters of adipic acid with low molecular weight alcohols, forexample with alcohols having from 1 to 4 carbon atoms, or ester mixturescomprising them, include reactants of any origin which, owing to theirpreparation method, comprise 1,4-cyclohexanediol and4-hydroxycyclohexan-1-one as impurities.

Preference is given to using for the process according to the inventionan ester mixture as obtained by esterifying a dicarboxylic acid mixturewhich comprises adipic acid, 6-hydroxycaproic acid,1,4-cyclohexanediols, 1,4-cyclohexanedione and 4-hydroxycyclohexan-1-oneand which is obtained as a by-product of the oxidation of cyclohexane tocyclohexanone/cyclohexanol with oxygen or oxygen-containing gases byextraction of the reaction mixture with water, with a low molecularweight alcohol, preferably n- or isobutanol, more preferably methanol(ester fraction a)).

The preparation of the dicarboxylic acid mixture, the ester fraction(referred to above as steps a) and b)) and the process for preparing1,6-hexanediol (steps c) and d)) is known and described in detail inDE-A 196 07 954 A and DE-A 196 07 955. The entire contents of thesedocuments are therefore incorporated by reference into the presentapplication.

Particular preference is given to using a mixture in the purifyinghydrogenation which is obtained by removing the excess esterificationalcohol and/or low boilers in a first distillation stage (alcoholremoval). Low boilers refer to by-products which have a lower boilingpoint than the desired esters, especially 1,2-cyclohexanediols,valerolactone, methyl 5-hydroxyvalerate, dimethyl glutarate, dimethylsuccinate. The alcohol removal is known as stage 3 from DE-A 196 07 955and DE-A 196 07 954, which are explicitly incorporated by reference.

The ester mixture obtained after the purifying hydrogenation is dividedby separation in a further distillation stage (stage b) of the processaccording to the invention), which is known per se and is described, forexample, as stage 4 in DE-A 196 07 954, into an ester fractionsubstantially free of 1,4-cyclohexanediols and a fraction comprising atleast the majority of the 1,4-cyclohexanediols. The ester fraction whichis substantially free of 1,4-cyclohexanediols is hydrogenated to thehexanediol.

The purifying hydrogenation may be carried out in the process accordingto the invention and in the processes disclosed in DE-A 196 07 954 andDE-A 196 07 955 directly after the esterification, i.e. before thealcohol removal or after the alcohol removal, more preferably after thealcohol removal.

The purifying hydrogenation is effected at from 20 to 300° C.,preferably from 50 to 200° C., more preferably from 100 to 160° C., andat a hydrogen pressure of from 1 to 200 bar, preferably from 1 to 100bar of H₂, more preferably at from 10 to 50 bar of H₂, with fixed bed,suspended catalyst or homogeneous catalysts.

The catalysts used for the purifying hydrogenation in the processaccording to the invention are preferably heterogeneous catalysts whichcontain at least one metal of group 8 to 12 of the Periodic Table, forexample ruthenium, osmium, iridium, platinum, palladium, rhodium, iron,copper, cobalt, nickel and zinc, and combinations of these metals. Thesemetals may be used either in the form of the pure metals or of thecompounds thereof, for example oxides or sulfides. Preference is givento using copper, nickel, cobalt, ruthenium or palladium catalysts. Thesecatalysts may be applied to the customary supports, for example TiO₂,Al₂O₃, ZrO₂, SiO₂, carbon or mixtures thereof. The thus obtainedsupported catalysts may be present in all known finishing forms.Examples are extrudates or tablets.

Preference is given to the use of supported palladium-, ruthenium-and/or copper-containing catalysts.

Suitable for use in the process of the present invention are Raneycopper, Raney nickel and Raney cobalt catalysts. These Raney catalystsmay be present in all known finishing forms, for example as tablets,extrudates or granules. Suitable Raney copper catalysts are, forexample, the Raney copper catalysts in the form of nuggets which aredisclosed by WO 99/03801, which is explicitly incorporated herein byreference. These catalysts have a particle size of the nuggets of from 2to 7 mm, a copper content of from 40 to 90% by weight, a Langmuirsurface area of from 5 to 50 m²/g, a copper surface area of from 0.5 to7 m²/g, an Hg pore volume of from 0.01 to 0.12 ml/g and an average porediameter of from 50 to 300 nm.

Also particularly suitable for use in the process according to theinvention is a catalyst which comprises ruthenium supported on shapedtitanium dioxide bodies, the shaped titanium dioxide bodies beingobtained by treating commercial titanium dioxide, before or after theshaping, with from 0.1 to 30% by weight of an acid in which titaniumdioxide is sparingly soluble, and which is used in the process accordingto the invention. Ruthenium may be used either in the form of the puremetal or in the form of a compound thereof, for example oxide orsulfide.

The catalytically active ruthenium is applied by processes known per se,preferably to prefabricated TiO₂ as the support material.

A titanium dioxide support suitable with preference for use in theruthenium-containing catalyst may be obtained in accordance with DE 19738 464 by treating commercial titanium dioxide, before or after theshaping, with from 0.1 to 30% by weight of an acid, based on titaniumdioxide, in which the titanium dioxide is sparingly soluble. Preferenceis given to using titanium dioxide in the anatase modification. Examplesof such suitable acids are formic acid, phosphoric acid, nitric acid,acetic acid or stearic acid.

The ruthenium active component may be applied in the form of a rutheniumsalt solution to the thus obtained titanium dioxide support in one ormore impregnation stages. Subsequently, the impregnated support is driedand, if appropriate, calcined. However, it is also possible toprecipitate ruthenium from a ruthenium salt solution, preferably withsodium carbonate, onto titanium oxide present as a powder in aqueoussuspension. The precipitated solids are washed, dried, calcined ifappropriate and shaped. In addition, it is also possible to convertvolatile ruthenium compounds, for example ruthenium acetylacetonate orruthenium carbonyl, into the gas phase and apply them to the support ina manner known per se (chemical vapor deposition).

The thus obtained, supported catalysts may be present in all knownfinishing forms. Examples are extrudates, tablets or granules. Beforethey are used, the ruthenium catalyst precursors are reduced by treatingwith hydrogenous gas, preferably at temperatures above 100° C.Preference is given to passivating the catalysts before they are used inthe process according to the invention with oxygenous gas mixtures,preferably with air-nitrogen mixtures, at temperatures of from 0 to 50°C., preferably at room temperature. It is also possible to install thecatalyst in the hydrogenation reactor in oxidic form and to reduce itunder reaction conditions.

The catalyst which is particularly preferred in accordance with theinvention has a ruthenium content of from 0.1 to 10% by weight,preferably from 2 to 6% by weight, based on the total weight of thecatalyst composed of catalytically active metal and support. Theinventive catalyst may have a sulfur content of from 0.01 to 1% byweight, based on the total weight of the catalyst (sulfur determination:coulometric).

The ruthenium surface area is from 1 to 20 m²/g, preferably from 5 to 15m²/g, and the BET surface area (determined to DIN 66 131) is from 5 to500 m²/g, preferably from 50 to 200 m²/g.

The inventive catalysts have a pore volume of from 0.1 to 1 ml/g. Thecatalysts also feature a cutting hardness of from 1 to 100 N.

If the activity and/or selectivity of the catalyst sink in the course ofoperation, the catalyst used in accordance with the invention may beregenerated by measures known to those skilled in the art. Thesepreferably include a reductive treatment of the catalyst in a hydrogenstream at elevated temperature. If appropriate, the reductive treatmentmay be preceded by an oxidative treatment. In this treatment, thecatalyst bed is flowed through at elevated temperature with a molecularoxygen-containing gas mixture, for example air. There is also thepossibility of washing the catalyst with a suitable solvent, for exampleethanol or THF, and subsequently drying it in a gas stream.

The hydrogenation may also be effected by literature hydrogenationreagents such as NaBH₄, LiAlH₄, etc., at from 20 to 200° C., preferablyfrom 50 to 200° C., more preferably from 100 to 160° C. Thehydrogenation may be carried out continuously and batchwise, preferablycontinuously.

The subsequent workup of the ester mixture obtained after the purifyinghydrogenation is effected as described in DE-A 196 07 954 A and DE-A 19607 955 for the ester stream which has not been subjected to anypurifying hydrogenation.

The hydrogenation of the esters (ester hydrogenation) to hexanediol iseffected in a manner known per se as described in DE-A 196 07 954, inparticular at from 20 to 300° C. and at a pressure of from 1 to 50 bar(in the case of hydrogenations in the gas phase over a fixed bedcatalyst) or at a pressure in the range of from 30 to 350 bar (in thecase of hydrogenations in the liquid phase with fixed bed or suspendedcatalyst). The hydrogenation may be carried out batchwise, preferablycontinuously.

The hydrogenation effluent of the ester hydrogenation consistssubstantially of 1,6-hexanediol and the esterification alcohol. Furtherconstituents are 1,5-pentanediol, 1,4-butanediol, 1,2-cyclohexanediolsand small amounts of monoalcohols having from 1 to 6 carbon atoms andwater.

The hydrogenation effluent of the ester hydrogenation is fed in the nextstage into, for example, a membrane system or preferably a distillationcolumn and separated into the esterification alcohol which additionallycomprises the majority of the further low-boiling components and astream which predominantly 1,6-hexanediol as well as 1,5-pentanediol. Ata pressure of from 10 to 1500 mbar, preferably from 30 to 1200 mbar,more preferably from 50 to 1000 mbar, top temperatures of from 0 to 120°C., preferably from 20 to 100° C., more preferably from 30 to 90° C.,and bottom temperatures of from 100 to 270° C., preferably from 140 to260° C., more preferably from 160 to 250° C., are attained.

The 1,6-hexanediol-containing stream is purified in a column. In thispurification, 1,5-pentanediol, the 1,2-cyclohexanediols and any furtherlow boilers present are removed overhead. When the 1,2-cyclohexanediolsand/or 1,5-pentanediol are to be obtained as additional products ofvalue, they may be separated in a further column. Any high boilerspresent are discharged via the bottom.

1,6-Hexanediol is withdrawn from a sidestream of the column in a purityof >99.5% with a distinctly reduced 1,4-cyclohexanediol content of from0.005 to 0.1% by weight.

At pressures of from 1 to 1000 bar, preferably from 5 to 800 mbar, morepreferably from 20 to 500 mbar, top temperatures of from 50 to 200° C.,preferably from 60 to 150° C., and bottom temperatures of from 130 to270° C., preferably from 150 to 250° C., are attained.

When only relatively small amounts of 1,6-hexanediol are to be prepared,the stages may also be combined in a batchwise fractional distillation.

The process according to the invention thus constitutes a simple andinexpensive procedure for obtaining highly pure 1,6-hexanediol withminimal amounts of 1,4-cyclohexanediols.

The examples which follow serve to illustrate the invention withoutrestricting it.

EXAMPLE 1 Batchwise Purifying Hydrogenation Example 1a

0.15 kg of ester mixture (approx. 90% by weight of dimethyl adipate and10% by weight of methyl hydroxycaproate) which contained about 1500 ppmof 1,4-cyclohexanedione was reacted with 10 g of a Pd/Al₂O₃ catalyst at130° C. and 30 bar of hydrogen for 180 min. The effluent containedapprox. 10% by weight of methyl hydroxycaproate, approx. 90% by weightof dimethyl adipate, 380 ppm of 1,4-cyclohexanediol, 200 ppm of1,4-cyclohexanedione and 1050 ppm of 4-hydroxycyclohexanone.

Example 1b

0.15 kg of ester mixture (approx. 90% by weight of dimethyl adipate and10% by weight of methyl hydroxycaproate) which contained about 1500 ppmof 1,4-cyclohexanedione was reacted with 10 g of a 2% Ru/Al₂O₃ catalystat 130° C. and 30 bar of hydrogen for 180 min. The effluent containedapprox. 10% by weight of methyl hydroxycaproate, approx. 90% by weightof dimethyl adipate, 1490 ppm of 1,4-cyclohexanediol, 0 ppm of1,4-cyclohexanedione and 0 ppm of 4-hydroxycyclohexanone.

Example 1c

0.15 kg of ester mixture (approx. 90% by weight of dimethyl adipate and10% by weight of methyl hydroxycaproate) which contained about 1500 ppmof 1,4-cyclohexanedione was reacted with 10 g of a 5% Ru/SiO₂ catalystat 130° C. and 30 bar of hydrogen for 150 min. The effluent containedapprox. 10% by weight of methyl hydroxycaproate, approx. 90% by weightof dimethyl adipate, 1720 ppm of 1,4-cyclohexanediol, 0 ppm of1,4-cyclohexanedione and 0 ppm of 4-hydroxycyclohexanone.

Example 1d

0.15 kg of ester mixture (approx. 90% by weight of dimethyl adipate and10% by weight of methyl hydroxycaproate) which contained about 1500 ppmof 1,4-cyclohexanedione was reacted with 10 g of a 5% Ru/TiO₂ catalystat 130° C. and 30 bar of hydrogen for 150 min. The effluent containedapprox. 10% by weight of methyl hydroxycaproate, approx. 90% by weightof dimethyl adipate, 1560 ppm of 1,4-cyclohexanediol, 0 ppm of1,4-cyclohexanedione and 0 ppm of 4-hydroxycyclohexanone.

Example 1e

0.15 kg of ester mixture (approx. 90% by weight of dimethyl adipate and10% by weight of methyl hydroxycaproate) which contained about 1500 ppmof 1,4-cyclohexanedione was reacted with 10 g of a 2% Ru/C catalyst at130° C. and 30 bar of hydrogen for 150 min. The effluent containedapprox. 10% by weight of methyl hydroxycaproate, approx. 90% by weightof dimethyl adipate, 1800 ppm of 1,4-cyclohexanediol, 0 ppm of1,4-cyclohexanedione and 0 ppm of 4-hydroxycyclohexanone.

Example 1f

0.15 kg of ester mixture (approx. 90% by weight of dimethyl adipate and10% by weight of methyl hydroxycaproate) which contained about 1500 ppmof 1,4-cyclohexanedione was reacted with 10 g of a 2% Ru/αAl₂O₃ catalystat 130° C. and 30 bar of hydrogen for 180 min. The effluent containedapprox. 10% by weight of methyl hydroxycaproate, approx. 90% by weightof dimethyl adipate, 2400 ppm of 1,4-cyclohexanediol, 0 ppm of1,4-cyclohexanedione and 0 ppm of 4-hydroxycyclohexanone.

Example 1e

0.15 kg of ester mixture prepared according to DE-A 196 07 954 (afterstage a); (approx. 5% by weight of dimethyl adipate and 16% by weight ofmethyl hydroxycaproate) which contained about 0.07% by weight of1,4-cyclohexanedione and 0.4% by weight of 4-hydroxycyclohexanone wasreacted with 10 g of a 5% Ru/TiO₂ catalyst at 150° C. and 30 bar ofhydrogen for 150 min. The effluent contained approx. 16% by weight ofmethyl hydroxycaproate, approx. 25% dimethyl adipate, 0 ppm of1,4-cyclohexanedione and 0 ppm of 4-hydroxycyclohexanone.

EXAMPLE 2 Continuous Hydrogenation, Before Stage b)

A C₆ ester mixture prepared according to DE-A 196 07 954 (after alcoholand low boiler removal before stage b); (approx. 25% by weight ofdimethyl adipate and 16% by weight of methyl hydroxycaproate) whichcontained about 0.07% by weight of 1,4-cyclohexanedione and 0.4% byweight of 4-hydroxycyclohexanone was converted continuously in a fixedbed reactor over a 2% Ru/TiO₂ catalyst which had been activatedbeforehand in a hydrogen stream at 180° C. Hydrogenation conditions:trickle bed, 250 ml of catalyst, 1.5 mm extrudates, feed 750-1500 g/h,no circulation, 30 bar, 150° C.). The effluent contained approx. 16%methyl hydroxycaproate, approx. 25% by weight of dimethyl adipate, <20ppm of 1,4-cyclohexanedione and 0 ppm of 4-hydroxycyclohexanone.

EXAMPLE 3 Preparation of 1,6-hexanediol (See FIG. 1) Stage 1:(Dewatering)

0.1 kg of dicarboxylic acid solution (adipic acid approx. 17% by weight,approx. 13% by weight of 6-hydroxycaproic acid, approx. 1.5% by weightof 1,4-cyclohexanediols, approx. 0.08% by weight of 1,4cyclohexanedione, approx. 45% water) was distilled continuously in adistillation apparatus (three-tray bubble-cap tray column with externaloil heating circuit, oil temperature 150° C., tray volume each approx.25 ml, feed via the bubble-cap trays) with attached randomly packedcolumn (approx. 4 theoretical plates, no reflux at the top). The topproduct obtained was 0.045 kg with a formic acid content in water ofapprox. 3%. In the bottom stream (5.5 kg), the water content was approx.0.4%.

Stage 2: (Esterification)

5.5 kg of the bottom stream from stage 1 were reacted with 8.3 kg ofmethanol and 14 g of sulfuric acid. The acid number of the effluentminus sulfuric acid was approx. 10 mg KOH/g.

Stage 3: (Alcohol Removal)

In a column, the esterification stream from stage 2 was distilled (1015mbar, top temperature 65° C. up to bottom temperature 125° C.). 7.0 kgwere drawn off via the top. 6.8 kg were obtained as the bottom product.

Stage 3a:

The bottom product from stage 3 was converted continuously in a fixedbed reactor over a 2% Ru/TiO₂ catalyst which had been activatedbeforehand in a hydrogen stream at 180° C. (Hydrogenation conditions:trickle bed, 250 ml of catalyst, 1.5 mm extrudates, feed 750 g/h, nocirculation, 30 bar, 150° C.). The effluent contained approx. 16% methylhydroxycaproate, approx. 25% dimethyl adipate, <20 ppm of1,4-cyclohexanedione and 0 ppm of 4-hydroxycyclohexanone.

Stage 4: (1,4-cyclohexanediol Removal)

In a 50 cm randomly packed column, the stream from stage 3a wasfractionally distilled (1 mbar, top temperature from 70 to 90° C. up tobottom temperature 180° C.). The 1,4-cyclohexanediols were obtained inthe bottoms.

0.6 kg was distilled off as low boilers (1,2-cyclohexanediols,valerolactone, methyl 5-hydroxyvalerate, dimethyl glutarate, dimethylsuccinate, inter alia); 4.3 kg were obtained as the fraction comprisingpredominantly dimethyl adipate and methyl 6-hydroxycaproate.

Stage 5: (Continuous Hydrogenation, Substream)

2.7 kg of C₆ ester mixture from stage 4 were hydrogenated continuouslyin a 25 ml reactor over a catalyst (catalyst, 60% CuO, 30% Al₂O₃, 10%Mn₂O₃) which had been activated beforehand in a hydrogen stream at 180°C. Hydrogenation conditions: feed 20 g/h, no circulation, 220 bar, 220°C.). The ester conversion was 99.5%, the 1,6-hexanediol selectivity wasabove 99%. Approx. 150 to 250 ppm of 1,4-cyclohexanediols are found inthe hydrogenation effluent.

Stage 6 and 7: (Hexanediol Purification)

2.5 kg of the hydrogenation effluent from stage 5 were fractionallydistilled (distillation still having attached 70 cm randomly packedcolumn, reflux ratio 2). At 1013 mbar, 0.5 kg of methanol was distilledoff and, after application of reduced pressure (20 mbar in stage 7),predominantly the 1,2-cyclohexanediols and 1,5-pentanediol distilledoff. Afterward (b.p. 146° C.), 1,6-hexanediol distilled off with apurity of >99.7%. The main by-product is approx. 200-300 ppm of1,4-cyclohexanediol.

Stage 8:

2.9 kg of the bottom effluent from stage 4 were admixed with 3.8 kg ofmethanol and 3.8 g of tetraisopropyl titanate and converted continuouslyin a 1 m-long tubular reactor of capacity 440 ml which was filled with 3mm V2A rings. The average residence time was approx. 2 h.

Stage 9:

The effluent from stage 8 was fractionally distilled analogously to theapparatus described in stage 3. At top temperature 65° C., 3.5 kg weredistilled off (predominantly methanol). 2.2 kg remained in the bottoms.

Stage 10:

The bottoms from stage 9 were fractionally distilled analogously tostage 4 up to a bottom temperature of 160° C. 1.3 kg were obtained asdistillate and can be hydrogenated directly or recycled into the 4^(th)stage. (Composition: 52% by weight of methyl 6-hydroxycaproate, 31% byweight of dimethyl adipate, 5% by weight of dimethyl glutarate, 4% byweight of methyl 5-hydroxycaproate and a multitude of further,quantitatively insignificant components).

Stage 11:

7 kg of the top product of stage 3 were fractionally distilled at 1015mbar on a 20 cm randomly packed column. 0.8 kg of first runnings wereobtained at top temperature from 59 to 65° C. and comprised, in additionto predominantly methanol, C₁-C₄-monoethyl esters. At top temperature65° C., 5.6 kg of methanol were obtained in a purity of >99.5%. Thebottoms (0.6 kg) consisted predominantly of water.

1. A process for preparing 1,6-hexanediol comprising hydrogenatingdialkyl adipates, alkyl 6-hydroxycaproates and 1,4-cyclohexanedione and4-hydroxycyclohexan-1-one as ester mixtures comprising impurities, theprocess further comprising: a) freeing the resulting esterificationmixture of excess alcohol and low boilers in a first distillation stage;b) carrying out a separation of the bottom product in a seconddistillation stage into an ester fraction substantially free of1,4-cyclohexanediols and a fraction comprising at least the majority ofthe 1,4-cyclohexanediols; c) catalytically hydrogenating the esterfraction substantially free of 1,4-cyclohexanediols; and d) in apurifying distillation stage, obtaining 1,6-hexanediol from thehydrogenation effluent in a manner known per se, which comprisesselectively hydrogenating the ester mixture before stage a) and/orbefore stage b) (purifying hydrogenation).
 2. The process according toclaim 1, wherein the purifying hydrogenation is carried out over aheterogeneous catalyst.
 3. The process according to claim 1, wherein thepurifying hydrogenation is carried out at from 20 to 300° C. and from 1to 200 bar of H₂.
 4. The process according to claim 1, wherein thepurifying hydrogenation is carried out over supported palladium-,ruthenium- and/or copper-containing catalysts.
 5. The process accordingto claim 1, wherein the purifying hydrogenation is carried out overruthenium supported on shaped titanium dioxide bodies.
 6. The processaccording to claim 1, wherein the reactant used is an ester mixture asobtained a) by esterifying with a low molecular weight alcohol adicarboxylic acid mixture which comprises adipic acid, 6-hydroxycaproicacid and small amounts of 1,4-cyclohexanediols and which is obtained asa by-product in the oxidation of cyclohexane tocyclohexanone/cyclohexanol with oxygen or oxygen-containing gases and bywater extraction of the reaction mixture (ester fraction a)).
 7. Theprocess according to claim 1, wherein the reactant used is an estermixture in which excess alcohol and low boilers have been removed fromester fraction a) (ester fraction b)).
 8. The process according to claim2, wherein the purifying hydrogenation is carried out at from 20 to 300°C. and from 1 to 200 bar of H₂.
 9. The process according to claim 2,wherein the purifying hydrogenation is carried out over supportedpalladium-, ruthenium- and/or copper-containing catalysts.
 10. Theprocess according to claim 3, wherein the purifying hydrogenation iscarried out over supported palladium-, ruthenium- and/orcopper-containing catalysts.
 11. The process according to claim 2,wherein the purifying hydrogenation is carried out over rutheniumsupported on shaped titanium dioxide bodies.
 12. The process accordingto claim 3, wherein the purifying hydrogenation is carried out overruthenium supported on shaped titanium dioxide bodies.
 13. The processaccording to claim 4, wherein the purifying hydrogenation is carried outover ruthenium supported on shaped titanium dioxide bodies.
 14. Theprocess according to claim 2, wherein the reactant used is an estermixture as obtained a) by esterifying with a low molecular weightalcohol a dicarboxylic acid mixture which comprises adipic acid,6-hydroxycaproic acid and small amounts of 1,4-cyclohexanediols andwhich is obtained as a by-product in the oxidation of cyclohexane tocyclohexanone/cyclohexanol with oxygen or oxygen-containing gases and bywater extraction of the reaction mixture (ester fraction a)).
 15. Theprocess according to claim 3, wherein the reactant used is an estermixture as obtained a) by esterifying with a low molecular weightalcohol a dicarboxylic acid mixture which comprises adipic acid,6-hydroxycaproic acid and small amounts of 1,4-cyclohexanediols andwhich is obtained as a by-product in the oxidation of cyclohexane tocyclohexanone/cyclohexanol with oxygen or oxygen-containing gases and bywater extraction of the reaction mixture (ester fraction a)).
 16. Theprocess according to claim 4, wherein the reactant used is an estermixture as obtained a) by esterifying with a low molecular weightalcohol a dicarboxylic acid mixture which comprises adipic acid,6-hydroxycaproic acid and small amounts of 1,4-cyclohexanediols andwhich is obtained as a by-product in the oxidation of cyclohexane tocyclohexanone/cyclohexanol with oxygen or oxygen-containing gases and bywater extraction of the reaction mixture (ester fraction a)).
 17. Theprocess according to claim 5, wherein the reactant used is an estermixture as obtained a) by esterifying with a low molecular weightalcohol a dicarboxylic acid mixture which comprises adipic acid,6-hydroxycaproic acid and small amounts of 1,4-cyclohexanediols andwhich is obtained as a by-product in the oxidation of cyclohexane tocyclohexanone/cyclohexanol with oxygen or oxygen-containing gases and bywater extraction of the reaction mixture (ester fraction a)).
 18. Theprocess according to claim 2, wherein the reactant used is an estermixture in which excess alcohol and low boilers have been removed fromester fraction a) (ester fraction b)).
 19. The process according toclaim 3, wherein the reactant used is an ester mixture in which excessalcohol and low boilers have been removed from ester fraction a) (esterfraction b)).
 20. The process according to claim 4, wherein the reactantused is an ester mixture in which excess alcohol and low boilers havebeen removed from ester fraction a) (ester fraction b)).